This is the procedure we use on our O-320 and have had noticeably smoother
and quicker starts since we started it:

RPM - 1200
Lean for maximum RPM rise. (Don't worry, no amount of leaning will hurt the
engine at this low power)
Run 20 seconds
Throttle - Quickly to Idle
Mixture - Idle Cut Off
Switches - OFF

Leaning for all ground operations is vital to keep the mags clear on this
engine. On startup and after exiting the active runway, set RPM to 1000 and
lean slowly for maximum RPM rise. This will often be about 100. Lean
slightly past until RPM just starts to fall. If the engine runs a hair
rough, that's OK. It will be lean enough that it will start to stumble and
quit if you try to run it faster than about 1300. This means you can't
accidentally take off with it leaned.

Since we started doing this, I have only had to clear a mag a couple of
times in two years.

Speaking of idle. Lubrication is poor on startup and especially critical in
this engine. The parts most susceptible to damage are the camshaft and
lifters. It is counter intuitive but, due to the valve train dynamics, the
stresses become less the faster the engine turns. Running at minimum idle
while it "warms up" actually is harder on the parts most likely to wear.
As soon as the oil pressure gets into the green, set the RPM to 1000 and
lean.

Keep the RPM at 1100 on the ground whenever this will not require rolling
use of the brakes. RPM over 1000 also gets combustion temperature high
enough to scavenge lead deposits and reduce plug fouling.

--
Roger Long